1. Field of the Invention
The present invention relates to a type of direct conversion used in a receiver etc. and more particularly to, a direct conversion receiver which can improve an image signal rejection ratio and, further, reduce the load of hardware and processing.
2. Description of the Related Art
In a configuration of a receiver employing a type of direct conversion, which is one of the conversion methods used in a receiver etc., when a modulated radio wave is received, almost the same frequency of a carrier wave signal (local frequency) as that of a desired reception frequency of this received signal is output by a local oscillator, so that the output signal from this local oscillator and the received signal are mixed to thereby convert a reception wave in the RF band directly into a base-band signal, which is then detected and demodulated.
By such a direct conversion receiver, however, since the frequency of a carrier wave output from the local oscillator is the same as that of a desired reception signal, such a DC (Direct Current) offset phenomenon occurs that the output from the local oscillator is input again to another input of the multiplication processing to be multiplied again by the output of the local oscillator to thereby offset a DC component of the base band signal or such a fundamental problem occurs that 1/f noise is generated because the intermediate frequency of the base band signal obtained by the multiplication is nearly 0 (zero), thus making it difficult to receive signals in a wide range of band.
To guard against this, there is available a direct conversion receiver employing the low IF (Intermediate Frequency) system.
By the direct conversion method of the low IF system, an offset frequency (difference in frequency) having such a extent as not to generate a DC offset or 1/f noise is given between a desired reception frequency and a frequency output by the local oscillator, to then convert the reception signal to a low IF, so that the reception signal is converted in frequency by the digital signal processing using the offset frequency, thus obtaining in-phase and quadrature outputs.
In the low IF system, however, it is necessary to suppress an image signal at an image frequency so that it may not overlap with the desired signal.
The above-mentioned conventional direct detection circuit has a problem that it is difficult to sufficiently attenuate at the band pass filter such an image frequency signal contained in a reception signal, with respect to a desired wave, so that it may sometimes be necessary to make variable the intermediate frequency, the band, etc. of the band pass filter.
That is, it is difficult to obtain wide band characteristics of a receiver provided with a direct conversion and so to implement a wide band low IF receiver.
To solve the above-mentioned problem of the image frequency signal, there is available such a method as to give processing way for rejecting the image signal.
By this method, for example, in a low IF-type direct conversion receiver, a desired reception signal is multiplied by a local-oscillator's frequency provided with an offset frequency to be down-converted into an low-IF signal, which then undergoes image signal rejection and then digital signal processing to convert the offset frequency, thus obtaining an in-phase output and a quadrature output.
Note here that one conventional technology related to a direct detection-system receiver is disclosed in Japan Patent Application Laid-Open No. Hei 10-70482 published on Mar. 10, 1998 (Applicant: Philips Electronics, Nemrose, Fennohtsharp, Inventor: Paul, Anthony, Moore, et al.)
This conventional technology provides an integrated receiver using a local oscillator which oscillates an intermediate frequency of a desired signal wave higher than a frequency of an interference signal wave directly detected to thereby down-convert the frequency of an input signal, thus removing the influence by the interference signal.
In a type of conventional low-IF direct conversion receiver, however, the image rejection processing section performs analog processing, so that there occurs an error in the coefficient of an analog filter etc. caused by fluctuations etc. in characteristics of the analog elements, thus giving rise to a problem of an insufficient image rejection ratio.
If, for example, an average value of the fluctuations in characteristics of the elements is 1%, the limit of the image suppression ratio by analog processing is 40 dB or so, thus giving rise to a problem of poor performance of image rejection.